1. Field of the Invention
The present invention relates to well cementing methods and compositions for use in cold subterranean environments, and more particularly, but not by way of limitation, to methods and compositions for cementing conductor strings in deep water offshore wells.
2. Description of the Prior Art
In carrying out completion operations in oil, gas and water wells, hydraulic cement compositions are commonly utilized. For example, hydraulic cement compositions are used in primary cementing operations whereby pipe such as casing is cemented in the well bore. That is, a hydraulic cement composition is pumped into the annular space between the walls of the well bore and the exterior of a pipe disposed therein. The cement composition is permitted to set in the annular space thereby forming an annular sheath of hardened impermeable cement therein. The objective of the cement sheath is to physically support and position the pipe in the well bore and bond the pipe to the walls of the well bore whereby the undesirable migration of fluids between zones or formations penetrated by the well bore is prevented.
Completion and remedial cementing operations carried out in wells at low temperatures, e.g., temperatures as low as about 40.degree. F., are particularly difficult as a result of the delay in the setting of the cement. That is, the lower the temperature to which a cement composition is subjected, the longer it takes for the cement composition to set. In order to allow a well cement composition time to be mixed on the surface and then pumped into a subterranean zone in a well before the cement composition develops gel strength and sets, a set retarding additive is commonly included in the cement composition. At low temperatures, the set retarding additives often delay the setting of cement compositions for times which are too long, even when only very small quantities of the set retarding additives are utilized, making the cost of the completion or remedial procedure prohibitive.
Another problem which occurs when a cement composition used in primary cementing is slow to set is the occurrence of pressurized formation fluid flow into and through the annulus before and after the cement composition sets. Such an occurrence is attributable to the inability of the cement composition to transmit hydrostatic pressure during the transition of the cement composition from a true fluid to a hard set mass. During the transition phase, initial hydration of the cement composition has begun and the slurry starts to develop static gel strength. While the cement composition has little or no compressive strength, it becomes partially self-supporting which lowers the hydrostatic pressure exerted by the composition on pressurized fluid containing formations penetrated by the well bore. That is, when a cement composition becomes partially self-supporting due to the development of gel strength prior to setting, volume reductions in the cement composition (caused by hydration and fluid loss) result in rapid decreases in the hydrostatic pressure exerted by the cement composition. When the pressure exerted by the cement composition falls below the pressure of formation fluids, the formation fluids enter the annulus and flow through the cement composition forming flow passages which remain after the cement composition sets and/or diluting the cement composition with water which prevents its ability to subsequently develop sufficient compressive strength and provide a competent seal.
The foregoing problems are aggravated in offshore wells which are completed in deep cold water. Such wells include conductor pipes which are cemented from the seafloor or mud line to a depth generally under about 2000 feet below the mud line. The formations between the mud line and about 2000 feet are usually relatively young geologically and are not well consolidated. The formations are largely a product of erosion from the continental shelf, and consequently, the formations readily fracture and often have abnormally high artesian water sand flow.
When cementing conductor string casing in the subterranean formation adjacent to the seafloor, the cold temperature of the cement composition after being pumped through the seawater causes the cement composition hydration to be slowed and the transition time to be extended, and as a result, the cement composition often allows the influx of water and other fluids into the annulus. These conditions can lead to cementing job failure, costly remedial work, and increased expense and rig time.
While cementing processes have been developed which improve the success rate of cementing conductor pipe casing in deep water offshore wells and in other low temperature applications, there is still a need for improved well cementing methods and compositions for use in such cold environments.